In polymer preparation, the production of the finished plastic molding composition, the compound, from the plastic raw materials with addition of fillers and reinforcing agents, plasticizers, coupling agents, lubricants, stabilizers etc. is termed compounding. Compounding takes place predominantly in extruders and comprises the process operations delivery, melting, dispersing, mixing, degassing and pressure build-up.
More recently, as low as possible a content of volatile components has been required in plastic mixtures. Degassing in the compounding of plastic raw materials is therefore increasingly important. Typically the level of volatile components to be degassed in the plastic raw materials is approximately 500 to 2000 ppm and the residual content in the finished compound less than 400 ppm.
In temperature-sensitive polymers or polymer blends, during the compounding process, apart from the residual monomers and solvents to be degassed which are already contained in the raw materials, monomers can also be formed by disassociation, for example with rubber-modified polycarbonate blends these are 1,3-butadiene and acrylonitrile. The disassociation kinetics is highly temperature-dependant, i.e. as the temperature rises disassociation increases over-proportionately. The difficulty in compounding is that for an effective degassing of the volatile components apart from a low pressure a high temperature is required, but at a high temperature disassociation at the same time greatly increases. In addition, it is made worse that with a reduction of the energy input to reduce the temperature and avoid disassociation, problems can occur with melting and blending of the compound components which has a negative effect on the product quality (i.e. the mechanical properties and the composition). This applies in particular at high throughput rates, thus short dwell times which are required for an economic compounding process. With a higher throughput, a good degassing of the volatile components is only economically achievable with the previous technology if at the same time an increase in the disassociation components is accepted.
The use of entraining agents in extruders is in fact known, though only of the degassing of polymer solutions with a solvent content greater than 1%. However, no simultaneous melting of solid components takes place because the polymer solution is added liquid and without additional dispersion of blend components and additives. While DE 199 14 143 A1 includes relevant disclosure no reference is made to the problematic disassociation of monomers (or oligomers). A process for the drying of polymer powders in which volatile constituents with a content of at least 1 wt. % are removed with melting of the polymer and optionally using an entraining agent, is also known from EP 0 768 157 A1. This technique is restricted to large-volume equipment and deals with the drying of certain polymers having high solvent-content. A use of entraining agents in the compounding of polymers with extruders, in particular of polycarbonate blends, is not known.
The present invention is directed to a process for producing temperature-sensitive polymer compounds having low residual content of volatile and disassociation components. In particular the process is directed to the production of rubber-modified polycarbonate blends.